1. Field of the Invention
This invention relates to electro-optical display devices of the type utilizing a thin layer of nematic or smectic mesomorphic liquid crystal composition, and in particular to packaging structures for such devices in which optically transparent polymer film or sheet substrates are utilized.
2. Description of the Prior Art
Liquid crystal display (LCD) devices utilize a liquid crystal material sandwiched between two substrates. Electrodes carried on the substrates are selectively energized by an electrical potential with the result that a turbulent flow of liquid crystal molecules occurs (light scattering type) or the molecules are oriented in a new direction (field effect type) such that the areas of the liquid crystal fluid which are subjected to the electrical field are observed to be lighter or darker than the background areas, when viewed in polarized light. The display devices are either reflective, in which case the back substrate is reflective and the front substrate is transparent, or transmissive in which case both substrates are transparent and the device is back lighted.
Only two major categories of liquid crystal materials are currently being used in LCD devices, one being the "nematic" state and the other being the "smectic" state. The smectic state is characterized as the most highly ordered state of liquid crystals. Materials of this type, however, do not exhibit many of the useful optical properties characteristic of the nematic state. Specifically, they do not exhibit "dynamic scattering" in response to an electric field. On the other hand, smectic materials are desirable in that they exhibit a very low crystal-to-mesomorphic transition temperature and often exist in the mesomorphic state at room temperatures.
Nematic liquid crystal materials frequently assume the characteristics of a thread-like texture when a thin section of the material is viewed between crossed polarizers. Nematic liquid crystals are normally transparent to light. However, when electric potential is imposed across the nematic liquid crystal layer, it causes the crystals to scatter light. This effect has been termed "dynamic scattering". The dynamic scattering effect of nematic liquid crystals in response to a voltage applied across the crystal is useful in many electro-optic devices such as alpha-numeric displays.
Conventional nematic liquid crystal materials exhibit the nematic mesophase generally at relatively high temperatures, and thus require external environmental control apparatus to maintain the material in a nematic mesophase. Further, the temperature range of the mesophase is generally very narrow, requiring that the temperature be accurately maintained. Lyotropic nematic liquid crystals have a relatively low crystal-to-mesomorphic transition temperature and are commonly used in the place of conventional thermotropic nematic liquid crystals.
Recently, several improvements have been made in liquid crystal display systems to improve their efficiency and operability. One such improvement is the use of twisted nematic liquid crystal composition sandwiched between optically transmissive plates. The internal surfaces of the plates that contain the liquid crystal composition, with the electrodes affixed thereto, can be conveniently prepared by unidirectionally rubbing the surfaces prior to assembling the device. By unidirectionally rubbing the internal surfaces of the plates, with the electrodes affixed thereto, and mounting the opposing plates with the axes of the rubbed surfaces being at ninety degree angles, the liquid crystal material will align itself in a twisted path that twists through a ninety degree angle between the surfaces. Thus, light will be rotated in a ninety degree angle as it passes through the twisted nematic liquid crystal composition from one surface of the cell to the other surface. Application of an electric field between the selected electrodes of the cell causes the twist of the nematic crystal composition to be temporarily removed in the portion of the cell between the selected electrodes.
By mounting an optical polarizer on the outside of the cell walls, parallel to the rub direction, polarized light can be passed through the wall of the cell and the polarized light will be rotated as it passes through the twisted liquid crystal composition. In such a liquid crystal cell, where the surfaces of the cell have been unidirectionally rubbed and mounted at ninety degree angles to each other, the polarized light will be rotated ninety degrees as it passes through the twisted liquid crystal composition. A second polarizer mounted on the outside of the second cell wall at ninety degrees to the first polarizer will allow the polarized light beam to pass through the second polarizer and be reflected by suitable reflector means mounted behind the second polarizer. The polarized light will then be reflected back into the liquid crystal material and will be twisted through ninety degrees as it is transmitted back through the internal portion of the liquid crystal cell and will exit through the first optical polarizer.
The characteristics of the substrate and/or electrode surfaces which are in intimate contact with the liquid crystal material can affect the orientation direction bias of tilting of the crystal in those areas. This is especially true with glass substrates for reasons that are unknown at this time. Also, in the reflective mode a nonconducting surface is needed upon which to form the electrode pattern which limits the choice of suitable substrates. Usually glass substrates having polarizers with backside mirror coatings are employed. The quality of the display is a function of the contrast between the electrically activated area and the background area.
Conventional liquid crystal display devices utilize glass as the substrate material. Fabrication of liquid crystal displays made from glass, in guage thicknesses less than 20 mils, is difficult because of the fragile nature of glass. When glass is used as a substrate, it must be protected in the electronic device by an unbreakable, transparent plastic window, which adds to the device cost. Moreover, the use of glass and the clear plastic window reduces the effective viewing angle because of the combined thickness of the layers. In an LCD, it is important to reduce the distance between the rear reflective polarizer and the liquid crystal layer to reduce the parallax between the "on" segment and the shadow of the "on" segment, thereby resulting in a wider viewing angle. The difference in refractive index of the liquid crystal material and glass substrates add another effect known as a "floating image" in conventional glass displays.
The development of large area liquid crystal display devices has been limited because the effective viewing angle diminishes directly in proportion to the thickness of the glass substrate. Moreover, glass contains ionic contamination, for example sodium ions, which corrupts the liquid crystal material causing increased battery power drain.
Optically clear polymer film such as mylar, polyethylene, triphtalate, poly-carbonate, poly vinyl chloride, cellulose triacetate, cellulose acetate, and cellulose butyrate have been proposed for use as substrates for encapsulating the liquid crystal material, for example, as shown in Culley et al U.S. Pat. No. 4,228,574. However, such materials, for example cellulose acetate and cellulose butarate, are chemically unstable, and are attacked by most organic solvents, acids and bases. Attempts to use such materials have failed because of their unstable nature in the presence of the liquid crystal material, and because of their incompatibility with environmental conditions such as temperature and humidity variations.